For safe storage of data carriers like CDs or DVDs commonly cassettes are used, into which the data carriers are put. The data carriers are being fastened to the housing of the cassette by means of a fastening device. The fastening device may either grip or fix the data carrier on its peripheral outer border, or, and this version is more commonly used, the fastening device comprises means to insert into the central opening of the data carrier (if need be in combination with an additional peripheral fixation). Such central fastening devices, usually comprising a crown of flexible tongues, which are elastically insertable into the central opening of the data carrier, are described in a large number of documents, such as for example in WO 01/39193, EP 0817196, EP 0762425, DE 4107218 or in U.S. Pat. No. 5,785,172.
Usually such cassettes are being produced by using thermoplastic materials, that is plastics with a flow transition above the usual temperature of use. Of particular interest in this respect are plastics which are available on the market for cheap prices, such as polyethylene (PE), particularly LDPE (Low Density Polyethylene) and HDPE (High Density Polyethylene), Polyvinylchloride (PVC), Polystyrene (PS) or Polypropylene (PP). When using such materials, usually the whole cassette is made from such a material in a forming process, it is however also possible to use combinations of different materials, wherein for example the outer part of a housing is made from one material, and the insert, i.e. the middle part, on which the fastening device is provided, is made from another material.
One of the problems in the context of such fastening devices is very often that such fastening devices have the tendency to break apart for example if a cassette is dropped down. In particular if Polystyrene is used for the fastening device, it is possible that if the cassette is dropped in a direction parallel to the plane of the CD fastened within the cassette, one or more of the tongues of the crown break out, the reason for which is that Polystyrene is stiff and rather brittle, wherein however this stiffness is desired because of the possible restoring force of the tongues. To avoid such breaking out of the tongues, the document EP 0692791 proposes to provide tongues which are undercut in the support region for the CD, wherein the tongues are unsupportedly extending comprising a radial arm and an axial arm, and wherein a continuous arched transition, or connection, respectively, is provided between the radial and the axial arm. This design allows a good fastening as well as a sufficient toughness of the tongues, because the tongues have an increased elasticity, and due to the undercut support the shearing forces at the tongues are well distributed.
Also problematic in the context of such fastening devices is the fact that if such a cassette is dropped, the data carrier very often jumps out of the fastening device and is subsequently loose in the cassette. This may, in particular if the loose data carrier is not noticed and the cassette with the loose data carrier is being moved, lead to irreversible damage of the data carrier. This problem can for example be solved in that the fastening device for the data carrier works with a larger force, which for example can be achieved by an increased stiffness, by means of additional peripheral fastening means, or by means of more pronounced retaining noses. All these measures however have the drawback that accordingly, to remove the data carrier from the cassette, an increased force has to be used, and that it is in particular more difficult to release the data carrier from the cassette, or take it out of the fastening device, respectively. This may even go so far that there is a realistic danger to damage the data carrier when removing it from the cassette.
Correspondingly, it would be desirable to have soft and thus usually less brittle material for the manufacture of such fastening devices. Unfortunately however, such soft materials can typically not be designed in a slender manner at the same time showing a sufficiently large restoring force of the tongues.